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Five field experiments were conducted to evaluate the relative attractiveness of
potential beneficial insectary plants to aphidophagous hover flies and parasitic
Hymenoptera and the effectiveness of interplanting selected flowering plants in a
broccoli field to enhance biocontrol of the cabbage aphid and green peach aphid.
In 1996 we established a preliminary screening trial to begin development of
our sampling methods and evaluations of the attractiveness of selected flowering
plants to hover flies and parasitic Hymenoptera.
In 1997, we conducted a field experiment at the Oregon State University
Vegetable Research Farm near Corvallis, OR to assess the relative attractiveness of 11
selected flowering plants to hover flies and parasitic Hymenoptera. Six of these
plants were also evaluated for attractiveness to aphidophagous hover flies in two on-farm
trials. The experimental design was a complete randomized block design, with
four replications at the OSU site, and three replications at the two on-farm sites.
Attractiveness of flowering plants to hover flies was assessed by conducting weekly
timed observations of feeding frequencies. Associations of parasitic Hymenoptera
were assessed by weekly timed vacuum sampling from a fixed area in plots of
flowering plants.
Attractiveness differed by dates and sites. Among early-season flowering
species, Coriandrum sativa (cilantro) was highly attractive to aphidophagous hover
flies and Brassica juncea (mustard), Fagopyrum esculentum (buckwheat) and
Agastache rugosa (Korean licorice mint) were most attractive to parasitic
Hymenoptera. Among late-season flowers, Achillea millefolium (yarrow), Foeniculum
vulgare (fennel) and Agastache rugosa (Korean licorice mint) were most attractive to
hover flies, but attractiveness to parasitic Hymenoptera did not differ.
An on-farm trial was conducted in 1997 at Stahlbush Island Farm near
Corvallis, OR. The objective of this trial was to test the hypothesis that interplanting
either alyssum (Lobularia maritima), or cilantro (Coriandrum sativa), with broccoli
(Brassica oleracea) would attract aphidophagous hover fly adults and parasitic
Hymenoptera, enhance oviposition in the adjacent crop, and increase larval predation
and parasitism in the adjacent crop, resulting in suppressed cabbage aphid
(Brevicoryne brassicae) and green peach aphid (Myzus persicae) populations in the
broccoli crop. The predominate hover fly species present were Toxomerus
occidentalis and T. marginatus. More adult female T. occidentalis were caught in pan
traps in plots with alyssum than in cilantro or control plots. More hover fly eggs
were found on broccoli leaves and a higher percent of the aphids present were
parasitized by Hymenoptera in plots with alyssum than in cilantro or control plots.
However, no differences in aphid intensities were found between treatment and
control plots. A comparison between the mean number of hover fly eggs found per
broccoli leaf and the mean number of aphid counted per broccoli leaf suggests there is
an association between the two. There appears to be an aphid density threshold
below which few hover fly eggs are laid. Gravid females were present in the crop
from the first sampling date on, yet hover fly eggs were not found in the crop until the
second to last sampling date. Our results indicate that the presence of alyssum
enhanced hover fly activity, but did not result in increased larval predation on aphids
in the crop.
In 1997 a survey of hover flies was conducted at each of the four experimental
sites. Hover flies were captured with sweep nets. Representative specimens were
identified to species by Christian Kassebeer, University of Kiel, Germany and
subsequent identifications were made from reference specimens and with taxonomic
keys. Twenty species were identified, 16 of which are aphidophagous. At the OSU
site and the two on-farm sites, where the relative attractiveness of flowering plants
was assessed, the six most common aphidophagous species, collected at all three sites,
were: Meliscaeva cinctella, Toxomerus marginatus, T. occidentalis, Sphaerophoria
sulphuripes, S. pyrrhina, and Scaeva pyrastri.